What is a VOC gas sensor, and how does the C-21 work?

Volatile organic compounds (VOCs) are carbon-based chemicals that evaporate readily at room temperature. In the workplace they come mostly from solvents, paints, thinners, inks, adhesives, degreasers, dry-cleaning fluid and fuel. Many have a TLV (threshold limit value) of only tens of ppm, and several are harmful well below the level at which most people can smell them.

The C-21 uses an HMOS (heated metal oxide semiconductor) sensor. The heated metal oxide surface changes its electrical resistance when VOC molecules react with it. The C-21 converts that change into a bargraph reading within a few seconds of the gas reaching the sensor. Air reaches the sensor by diffusion, so there is no pump to service.

Green = normal · Yellow = caution · Red = danger. By default the alarm and relay trigger at the first red bar.

Which VOCs does the C-21 detect?

The C-21 responds to most solvent vapours. The table gives Eco Sensors' typical detection levels (Application Note VO-102) for common workplace VOCs, alongside the 8-hour limits in South Africa's Regulations for Hazardous Chemical Agents, 2021. The C-21 is a good fit where its alarm level sits at or below the exposure limit, and a poor fit where the limit is far below what the sensor can see.

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TLV values are the figures printed in Eco Sensors' application note. Current ACGIH TLVs and South African OELs may differ, so always check the limit that applies to your site. The SA column lists the 8-hour TWA from Annexure 2 of the Regulations for Hazardous Chemical Agents, 2021 (OEL-ML = maximum limit, OEL-RL = restricted limit).

C-21 VOC gas sensor specifications

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Where the C-21 is used, and what it does there

How to choose a VOC detector: is the C-21 the right one?

Answer these four questions before buying any VOC instrument:

1. Which compound, and what is its exposure limit? If your main solvent's 8-hour limit is at or above the C-21's first-red-bar level (for example toluene, MEK, acetone or perc), the C-21 suits it. If the limit sits between the first-detect and alarm levels (trichloroethylene in SA), lower the set point. If the limit is around 1 ppm or lower (benzene, formaldehyde), you need a more sensitive, compound-specific method.
2. Do you need a warning or a number? For a clear go / caution / stop signal that anyone on the floor can read, choose the C-21. If you need a ppm figure for an exposure report, you need a quantitative instrument or a laboratory method.
3. Fixed, portable, or both? The C-21 does both: on the adapter it runs continuously as a fixed alarm, and on battery it becomes a handheld leak tracer.
4. Does it need to control anything? The relay output can switch an external alarm, a strobe or a fan-control relay, and the data logger output records levels over a shift.

VOC detection technologies compared

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Most sites combine them: periodic exposure measurements for compliance, plus a C-21 running every day to catch the leak, spill or extraction failure that happens between surveys.

Set-up, placement and calibration

Placement

• Light solvents (acetone, alcohols, MEK, toluene) disperse quickly through a room, so mount the C-21 at breathing height in the work area.
• Heavy chlorinated solvents (perchloroethylene, methylene chloride, trichloroethylene) sink to the floor and bench tops and cling to fabrics. Mount the sensor low and close to the source.
• Don't rely on smell. Odour thresholds vary widely between people, and some VOCs are hard to smell at harmful levels.

Alarm set point and relay

From the factory, the alarm and relay trigger at the first red bar. To change this, open the case and set the 8-position DIP switch near the board edge to any of the top 8 bars, from the 3rd green bar up to the 3rd red bar. Only one switch should be on at a time. The relay is rated 30 V, 130 mA AC or DC. To drive an extraction fan or other mains load, Eco Sensors advises placing a power-handling relay between the C-21 contacts and the load. A 12 V DC relay coil can be powered from the C-21's adapter.

Calibration

Let the sensor warm up for at least 15 minutes. Then present a known concentration of the target gas at about 0.1 L/min, using a certified gas mixture, a diluted liquid standard in a glass syringe, or a calibrated reference instrument. Adjust the span potentiometer behind the rear cover until the reading matches a convenient bargraph point, such as the first red bar. Use glass syringes and non-rubber seals, because some solvents cling to plastics and rubber. For a quick daily function check, Eco Sensors notes that a felt-tip marker held about 5 mm from the sensor grille should drive the bargraph to the top.

VOC monitoring and South African regulations

In South Africa, workplace exposure to solvents falls under the Occupational Health and Safety Act, 1993 and the Regulations for Hazardous Chemical Agents, 2021 (GN R280, Government Gazette 44348). The regulations set two kinds of occupational exposure limit:

• OEL-ML (maximum limit), Annexure 2 Table 2: includes benzene, formaldehyde, dichloromethane and trichloroethylene, several of them flagged as carcinogens.
• OEL-RL (restricted limit), Annexure 2 Table 3: includes toluene, acetone, MEK, diacetone alcohol and perchloroethylene.

Where a risk assessment shows that employees may be exposed, the regulations require air monitoring, with representative measurements at least every 24 months for agents with an OEL-ML or OEL-RL. That periodic survey shows whether controls worked on the day it was done. A C-21 running continuously shows whether they are working today, and alarms when extraction fails, a drum is left open or a seal starts to leak.

Ecotao supplies the C-21 from Cape Town to workshops, dry cleaners, printers and manufacturers across South Africa, and delivers to Namibia, Botswana, Zimbabwe, Mozambique, Lesotho and Eswatini. For ozone-based VOC and odour treatment, see our air ozone monitors and controllers and the full ozone sensors & instruments range.